Nathan-Baker effect

How may one establish experimentally the existence of hyperconjngation Measuring the properties indicated (the charge and the C-H bond length) wonld seem to be a simple and direct approach. However, neither of these things is really snsceptible to direct measurement in practice. There just are no available procedures of the required accuracy to make the determinations that we would like to have. So chemists tried to settle the problem by other kinds of experiments. But the kinds of experiments that were available, especially in the 1930s and for many years after that, were quite indirect. Here are the kind of reasoning and experiments that were employed. 

If there is hyperconjugation, however, it can simultaneously lead to resonance structures wherein electrons are removed from the attached hydrogen (or other group) and pushed into the 7t-electron system. One can write resonance forms for this hyperconjugation as in Structure 7  

Evidently the f-butyl group donates electrons into the n system of the benzene ring more so than the methyl group does, hence the larger (gas-phase) dipole moment of the former. 

To the extent that hyperconjugation leads to a stabihzation of that carbocation, the pertinent resonance forms for the ions would be as shown in Structures 9 and 10  

There are problems with this kind of experiment, however. For one thing, the reactions are carried out in a solvent, which then solvates the carbocations formed at the transition state of the reaction. The difference in the solvation energies between the methyl- and t-butyl ions may be substantial, and this would upset the comparison. But what seems to be even more important is this. This experiment only measures the difference in the amount of hyperconjugation in the two cases (methyl vs. t-butyl), not the absolute value. It is certainly true that CH3 is a very unstable ion. But H is not all 

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This came to be called the Baker-Nathan effect and has since been found in many processes. Baker and Nathan ° explained it by considering that hyperconjugative forms contribute to the actual structure of toluene ... 

However, the Baker-Nathan effect has now been shown not to be caused by hyperconjugation, but by differential solvation. This was demonstrated by the finding that in certain instances where the Baker-Nathan effect was found to apply in solution, the order was completely reversed in the gas phase. ° Since the molecular structures are unchanged in going from the gas phase into solution, it is evident that the Baker-Nathan order in these cases is not caused by a structural feature (hyperconjugation) but by the solvent. That is, each alkyl group is solvated to a different extent. 

Note Had you been given the choice, CH3Br, you would not choose it due to a possible Baker-Nathan effect (no-bond resonance). In base, one result is protolysis ... 

The cumyl cation (4) has been the subject of an X-ray crystallographic study, as its hexafluoroantimonate salt at —124 °C.31 It is nearly planar (8 ° twist), with a short bond between the C+ and the ring (1.41 A), consistent with benzylic delocalization. The Me—C+ bonds are also shortened, indicative of hyperconjugative interaction.31 However, calculations are taken to show that hyperconjugation is not important in isolated benzyl cations e.g. structures such as (6) are not important contributors to the overall structure of (5).32 The stabilization provided by alkyl groups would thus be because of their polarizability, and the Baker-Nathan effect would be due to steric hindrance to solvation.32 The heats of formation of some a-mcthylbcnzyl cations indicate that the primary stabilization in these species comes from the a-substitucnts, and that the stabilization provided by the aromatic ring is secondary.33... 

While the effect on oxidation-reduction potentials of substituents on phenanthroline ligands is regular, studies of the oxidation of [Os(bipy)(terpy)X] + species, where X is an alkyl-substituted pyridine molecule (111), do not show a linear dependence of E° on the pA of X. These results have been explained in terms of the Baker-Nathan effect. However, taken in conjunction with the entropy data of Kratochvil and Knoeck (439) for substituted iron complexes, an explanation involving changes in solvation with substituents seems preferable. The potentials of various Os(II)/Os(III) couples (111) and Ru(II)/Ru(III) couples (220) have been used to study the effect of the overall charge on the... 

Baker-Nathan effect. Effect originally observed in the reaction of p-substituted benzyl bromides with pyridine and other processes in which the observed rates are opposite to those predicted by the electron-releasing inductive effect of alkyl groups, i.e., CH3 > CH3CH2 > (CH3)2CH > C(CH3)3. To explain it, a type of electron delocalization involving 2 electrons was proposed, termed hyperconjugation, which manifests itself in systems in which a saturated carbon atom attached to an unsaturated carbon or one with an empty orbital bears at least one hydrogen atom. 

It is with no disrespect to the late Professor Coulson s very good friend, Professor R. S. Mulliken, that we quote Coulson s affectionate remark in the lecture This used to be known as the Baker-Nathan effect but later, thanks to Mulliken s ability to invert names, as hyperconjugation". 

In the low-absorptivity region, where the saturation effect is insignificant, the electronic and vibrational contributions to the absorptivity become comparable (Ballester et al., 1964b) and consequently acceptable correlations are obtained only in a very few instances, such as in alkylbenzenes. Actually, the square law has allowed the evaluation of the carbon-carbon and carbon-hydrogen bond hyperconjugation (Baker-Nathan effect) (Ballester and Riera, 1964a March, 1985, p. 65). 

For a discussion of the role of polarizibility in donor abihty, see (a) Taft, R. W., Topsom, R. D. (1987). The Nature and Analysis of Substitutent Electronic Effects. Progress in Physical Organic Chemistry, 16, 1-83 (b) Exner, O., B6hm, S. (1997). Baker-Nathan effect, hyperconjugation and polarizability effects in isolated molecules. Journal of the Chemical Society, 6, 1235-1240. 

The relative rates of S Ar reactions of alkylbenzenes are opposite to those expected from the electron release effect of the alkyl groups (Baker-Nathan effect) (16). The rationalization of this phenomenon in terms of the HSAB principle is as follows. The stability of the a-complex intermediate may be... 

To summarize Baker and Nathan observed that hyperconjugation appeared to occur to a greater extent when H was the nonbonded fragment in the sacrificial valence bond form, compared with the case where CHJ was the fragment. This difference between the two (called the Baker-Nathan effect) was not very great in any case and was subsequently shown to be due to a solvation effect. This has led to the statement... 

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